This invention is concerned with a process for the preparation of chrysanthemic acid esters and their homologues.
This invention is more specifically concerned with a process for the preparation of chrysanthemic acid esters and their homologues of the formula: ##STR4## in which
R.sub.1 and R.sub.2, which may be the same or different, are hydrogen or an alkyl group having 1 to 6 carbon atoms, preferably methyl;
R.sub.3 and R.sub.4, which may be the same or different, are an alkyl group having 1 to 6 carbon atoms, preferably methyl, or a halogen atom, preferably fluorine, chlorine or bromine, or, together with the carbon atom to which they are attached, form a cycloalkyl group; and
R.sub.5 is an alkyl group having 1 to 6 carbon atoms, preferably ethyl or propyl.
The compounds of formula I are valuable intermediates for the preparation of synthetic pyrethroids having insecticidal properties of the kind described for example in Nature, 246, 16th Nov. 1973, pages 169-170.
Processes for making some of the compounds of formula I and the corresponding acids (R.sub.5 = H) are known.
Thus, chrysanthemic acid (I, R.sub.1 = R.sub.2 = CH.sub.3, R.sub.3 = R.sub.4 = CH.sub.3, R.sub.5 = H) has been prepared by hydrolysis of pyrethrins of natural origin or by the synthesis of Staudinger et al, Helv. Chim. Acta (1924) 7, p 390, further developed by Campbell et al, J. Chem. Soc., (1945), p 283. This synthesis, which starts with the reaction of ethyl diazoacetate with 2,5-dimethyl-hexa-2,4-diene and leads to a mixture of the (dl)-cis- and (dl)-trans-chrysanthemic acids, is, however, rather difficult to carry out because of the instability of ethyl diazoacetate. This instability makes the industrial use of this process very difficult. A similar synthesis, in which ethyl diazoacetate is replaced by diazoaceto-nitrile, gives pure (dl)-trans-chrysanthemic acid, but involves even greater risks.
U.S. Pat. No. 3,077,496 describes a process for the preparation of (dl)-trans-chrysanthemic acid in which 4-methyl-3-isobutenyl-.gamma.-valerolactone is converted into alkyl 5-methyl-3-(1-halogeno-isopropyl)-hex-4-enoate, followed by cyclisation of this ester into a crysanthemic acid ester, and saponification.
U.S. Pat. No. 3,354,196 describes converting intermediates of the formula ##STR5## in which R is hydrogen or methyl, Y is --CN or --COOR', and R' is a lower alkyl radical, into intermediates of the formula in which X is a reactive ester residue, such as a halogen atom or a sulphonic ester residue, or an aliphatic or aromatic carboxylic ester residue, and then converting the intermediates IV to compounds of the formula: ##STR6## which are, in turn, converted to chrysanthemic acid or its lower homologue (in which R = H) by saponification.
U.S. Pat. No. 3,652,652 describes a process using 4-methyl-3-(2-methallyl)-.gamma.-valerolactone as the starting compound; this compound is reacted with any halogenating agent capable of effecting ring cleavage to form a .gamma.-halogeno-acyl halide. The compound obtained is reacted with an alkanol. Depending on the reaction conditions, that is upon whether or not any hydrogen halide is evolved during the initial halogenation step and upon whether or not a hydrogen halide is present with the alkanol in the second step, one or both of the following compounds VI and VII are obtained: ##STR7## where Hal is a halogen atom and R is an alkyl radical.
By cyclising under suitable conditions, the intermediate VI can be directly converted into a chrysanthemic acid ester. The intermediate VII can be cyclised to form the cyclopropane ring and the product then isomerised by heating in the presence of toluene-p-sulphonic acid or other organic sulphonic acid to shift the double bond and thus obtain a chrysanthemic acid ester.